Move oven rack to middle position and preheat to 200 degrees Place the popped popcorn in a large foil roasting pan.
In a large saucepan over medium heat add the butter, brown sugar, salt, corn syrup, and vanilla. Stir occasionally until everything is melted together and comes to a boil. Continue to stir while the mixture boils for 5 minutes. Take the saucepan off of the heat and stir in the baking soda.
Pour the caramel mixture over the popcorn and stir until the popcorn is coated in the caramel.
Bake for 1 hour, stirring every 15 minutes.
Line the countertop with waxed paper and pour the popcorn onto the paper. Spread it out and let the caramel corn cool and dry for at least 15 minutes before eating.